XRPFees: Fee setting and handling improvements (#4247)

* Introduces amendment `XRPFees`
* Convert fee voting and protocol messages to use XRPAmounts
* Includes Validations, Change transactions, the "Fees" ledger object,
  and subscription messages

* Improve handling of 0 drop reference fee with TxQ. For use with networks that do not want to require fees
* Note that fee escalation logic is still in place, which may cause the
  open ledger fee to rise if the network is busy. 0 drop transactions
  will still queue, and fee escalation can be effectively disabled by
  modifying the configuration on all nodes

* Change default network reserves to match Mainnet

* Name the new SFields *Drops (not *XRP)
* Reserve SField IDs for Hooks

* Clarify comments explaining the ttFEE transaction field validation

src/ripple/protocol/impl/STValidation.cpp

@@ -58,9 +58,14 @@ STValidation::validationFormat()
         {sfSigningPubKey,       soeREQUIRED},
         {sfSignature,           soeREQUIRED},
         {sfConsensusHash,       soeOPTIONAL},
+        // featureHardenedValidations
         {sfCookie,              soeDEFAULT},
         {sfValidatedHash,       soeOPTIONAL},
         {sfServerVersion,       soeOPTIONAL},
+        // featureXRPFees
+        {sfBaseFeeDrops,          soeOPTIONAL},
+        {sfReserveBaseDrops,      soeOPTIONAL},
+        {sfReserveIncrementDrops, soeOPTIONAL},
     };
     // clang-format on